For example, a related-art abnormality determination device of Patent Literature 1 determines that a rotation angle detection device is abnormal when a square root of a sum of squares of a sine wave signal and a cosine wave signal is outside of a predetermined range.
Further, a failure detection circuit of Patent Literature 2 determines abnormality when a value obtained by adding a signal +sin θ and a signal −sin θ or a value obtained by adding a signal +cos θ and a signal −cos θ exceeds a threshold value.